Insert different type of entities in GAE using JPA - google-app-engine

Question

I am new to GAE and Datastore
I am trying to insert different types of entities in GAE Datastore using JPA.
For example i have to insert Employee,EmployeePersonalInfo(Having a Employee Key),EmployeeAddressInfo(Having a EmployeePersonalInfo key). Here i am not creating any foreign key relationships between entities.My entities will looks like follows
public class Employee{
private String name;
private Key key;
}
public class EmployeePersonalInfo{
private String emailAddress;
private Key key;
private Key employeeKey;
}
public class EmployeeAddressInfo{
private String cityName;
private Key key;
private Key employeePersonalInfoKey;
}
i am trying to insert around 5 record on each table like
public class EmployeeController{
/*This method will be called for 5 times*/
public void insertEmployeeDetails(Employee emp,EmployeePersonalInfo perInfo,EmployeeAddressInfo addressInfo){
employeeDao.save(emp);
employeePersonalInfoDao.save(perInfo);
employeeAddressInfoDao.save(addressInfo);
}
}
Every time When a call goes to Save method of the DAO classes i will open the EntityManager object , and close after the operation.
public void save(Employee emp){
EntityManager em = EMFService.get().createEntityManager();
em.save(emp);
em.close();
}
Here sometimes i am getting an exception like
Caused by: java.lang.IllegalArgumentException: cross-group transaction need to be explicitly specified, see TransactionOptions.Builder.withXG
I have seen my so many solutions to this problem but i am not able to understand what is the real problem and solution. Please help me